Lipid biosynthesis in green leaves of developing maize

Successive leaf sections from the base to the tip of rapidly developing leaves of 7-day-old maize (Zea mays var. Kelvedon Glory), grown in the light, utilized acetate for fatty acid biosynthesis in a very divergent manner. Basal regions of the leaf containing proplastids synthesized insignificant proportions of unsaturated fatty acids and appreciable proportions of fatty acids with 20 or more carbon atoms. An increase in the light intensity during incubations with acetate-1-¹⁴C resulted in very little enhancement of either total or polyunsaturated fatty acid biosynthesis in this tissue.     

Plastid differentiation, acyl lipid, and Fatty Acid changes in developing green maize leaves

Plastid differentiation, acyl lipid, and fatty acid composition have been followed in successive 2-cm sections from the base (youngest tissue) to the tip (oldest tissue) of green Zea mays (maize) leaves grown under a normal diurnal light regime. Although the youngest cells (0-4 cm from the leaf base) had only proplastids with one or two grana, they contained chlorophylls a and b, monogalactosyldiglyceride, digalactosyldiglyceride, sulfolipid, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol. In the more mature sections, the plastids increased in size 5-fold, and differentiation into mesophyll and bundle-shealth chloroplasts had occurred. Concomitantly, the levels of all the lipids increased with the exception of phosphatidylcholine and phosphatidylethanolamine which decreased. With increasing cell maturity, the percentage of linolenic acid increased in all the individual acyl lipids, but palmitic acid remained constant in phosphatidylcholine, phosphatidylethanolamine, and sulfolipid. The Δ^3t-hexadecenoic acid was only detectable in the phosphatidylglycerol of the most mature maize tissue.     

Lipid metabolism in green leaves of developing monocotyledons

Lipid synthesis was studied in successive leaf sections from the base to the tip of developing barley (Hordeum vulgare L.), maize (Zea mays L.), rye grass (Lolium perenne L.) and wheat (Triticum aestivum L.) leaves. The endogenous levels of acyl lipids and their constituent fatty acids from the same leaf sections were also analysed. The principle chloroplast acyl lipids showed a relative increase in amount with the age of the leaf section. Their content of -linolenic acid also increased whereas there was little change in the amount of this acid in phosphatidylcholine and phosphatidylethanolamine, which are primarily non-chloroplastic. The content of trans-3-hexadecenoic acid in phosphatidylglycerol increased approximately 20-fold between the youngest (basal) and oldest (distal) leaf sections.The incorporation of [¹⁴C]acetate was always high into monogalactosyldiacylglycerol, phosphatidylcholine and the neutral lipid (mainly pigments) fractions. With increasing age, the neutral lipids were less well labelled. In three of the plant species but not in barley, phosphatidylglycerol was heavily labelled. Monogalactosyldiacylglycerol usually contained the highest amount of radioactivity in the middle leaf sections. Apart from these generalisations, each plant type had its own specific pattern of radiolabelling.     

Isoenzyme of pyruvate kinase in proplastids from developing castor bean endosperm

Proplastids from developing castor bean (Ricinus communis) endosperm have a pyruvate kinase activity which is extremely unstable on isolation from the organelle. It can be stabilized by 20 mm 2-mercaptoethanol in 20% ethylene glycol. In contrast the soluble pyruvate kinase is stable at 60 C for 10 minutes. The two activities have different pH optima. The soluble and the proplastid activities are eluted from a diethylaminoethyl-Sephadex A-25 sievorptive column at different ionic strengths.     

Polysomal changes in developing wheat leaves

Summary When leaf sections of 7-day old dark grown wheat leaves were incubated in white light, they unrolled and greened. Gibberellic acid was able to replace the light requirement and abscisic acid (ABA) inhibited the response to light. The percentage of ribosomes occurring as polysomes increased in response to light but not in response to GA₃ treatment. Although ABA inhibited the unrolling and greening in light, it did not cause a preferential decrease or inhibition of polysome formation.     

The appearance of photosynthetic proteins in developing maize leaves

Soybean (Glycine max (L.) Merr.) protoplasts have been surface-labelled with cationized ferritin, and the fate of the label has been followed ultrastructurally. Endocytosis of the label occurs via the coated-membrane system. The pathway followed by the label, once it has been taken into the interior of the protoplast, appears to be similar to that found during receptor-mediated endocytosis in animal cells. Cationized ferritin is first seen in coated vesicles but rapidly appears in smooth vesicles. Labelled, partially coated vesicles are occasionally observed, indicating that the smooth vesicles may have arisen by the uncoating of coated vesicles. Structures which eventually become labelled with cationized ferritin include multivesicular bodies, dictyosomes, large smooth vesicles, and a system of partially coated reticula.Abbreviation CF cationized ferritin     

Ultrastructural detection of sucrose synthase distribution in developing maize leaves

Summary A developing maize leaf grows by the activity of a basal meristematic region and an adjacent elongating zone, resulting in a morphological and functional gradient along the leaf. We have used this system to detect the spatial and temporal expression of an enzyme, sucrose synthase, which plays a pivotal role in the sucrose import-export transition which occurs along a monocotyledon leaf. Immunogold labeling was used to detect the cellular and sub-cellular distribution of sucrose synthase (SS) at the electron microscopical level; the protein was visualized using a polyclonal antiserum on embedded tissue sections. Immunolabel was observed in the cytosol of dividing meristematic cells, expanding cells of the elongation zone, and in differentiating cells of young photosynthetic tissue. In fully differentiated leaf tissue, however, the protein was no longer immuno-detectable in photosynthetic cells, but was present in the guard and subsidiary cells of stomata and in companion cells within the phloem tissue of vascular bundles. The tissue- and cell-specific localization of sucrose synthase changes along the growing leaf as a function of the developmental state and the associated need for sucrose import or export.     

The formation of chlorophyll from chlorophyllide in leaves containing proplastids is a four-step process

The time course of the different esters of chlorophyllide (Chlide) during the formation of chlorophyll a (Chl) in embryonic bean leaves containing proplastids was investigated by HPLC. After the reduction of photoactive Pchlide (Pchlide) to Chlide, three intermediates, i.e. Chlide geranylgeraniol, Chlide dihydrogeranylgeraniol and Chlide tetrahydrogeranylgeraniol were detected before the formation of Chlide phytol, i.e. authentic Chl. The transformation of Chlide to Chl was found to be much faster in leaves containing proplastids than in etiolated leaves with etioplasts.     

Photosynthetic gene expression and cellular differentiation in developing maize leaves

We have exploited the positional gradient of cellular differentiation in Zea mays leaves to study the accumulation of mRNAs encoding subunits of the two CO₂-fixing enzymes and the major chlorophyll-binding protein. These three proteins are differentially compartmentalized in the two photosynthetically active cell types of the leaf. Previous studies have shown that accumulation of the two carboxylases commences 2 to 4 cm from the base of the leaf (Mayfield SP, WC Taylor Planta 161: 481-486) at a position where bundle sheath and mesophyll cells show morphological evidence of maturation. The light-harvesting chlorophyll a/b protein accumulates progressively from the leaf base, as does its mRNA, in spite of its localization in mesophyll cells after cellular differentiation occurs. While small quantities of phosphoenolpyruvate carboxylase mRNA are detectable in the basal region of the leaf, significant mRNA accumulation is coincident with that of the polypeptide at 4 to 6 cm from the leaf base, the region where bundle sheath and mesophyll cells exhibit fully differentiated morphologies. mRNAs encoding the small and large subunits of ribulose 1,5-bisphosphate carboxylase accumulate to significant levels before bundle sheath cells are fully differentiated and before their polypeptides are detectable. Cytological examination indicates that this is the position at which the maturation of intermediate vascular bundles is first evident. Cytosolically localized small subunit mRNA and chloroplast-localized large subunit mRNA are complexed with polyribosomes at all positions of the leaf.     

Cytological changes in palisade cells of developing sunflower leaves

Summary Changes in the relative volume of palisade cells (V_v) allocated to various organelle compartments during postemergent leaf development was measured using stereological techniques. The surface to volume ratios (S_v) of the chloroplast and mitochondrial membranes were also measured at each stage. Three leaf stages were sampled, each was defined based on lamina length (10, 45, and 150 mm). The last stage represented a fully expanded leaf. Chloroplast and nuclear compartment V_v values changed significantly in the early stages when cells were actively dividing. Mitochondrial and vacuolar compartment V_v values showed significant changes in the latter stages during cell expansion. The oil vesicle and microbody compartments showed no significant change in V_v value during the developmental process. The surface to volume ratios of the chloroplast membranes increased significantly throughout all stages of the leaf development while mitochondrial cristae S_v values did not change. Organelle replication rates appeared to be independent of changes in cell volume with each organelle exhibiting a specific replication activity pattern. The results of this study suggest two possible mechanisms for the control of cell structural development involving both intrinsic and extrinsic factors.     

Carbon dioxide fixation and related properties in sections of the developing green maize leaf

Light and dark ¹⁴CO₂ assimilation, pulse-chase (¹⁴CO₂ followed by ¹²CO₂) labeling experiments both in the light and in the dark, photorespiratory activity and some enzymes (ribulose 1,5-bisphosphate (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, and NADP-malic enzyme) were followed in sections of 2.5 centimeters from the base (younger tissue) to the tip (oldest tissue) of the green maize leaf. Tissue was taken from the third leaf of 12- to 16-day-old plants consisting of sections 0 to 2.5 centimeters (base), 4.5 to 7.0 centimeters (center) and 9.0 to 11.5 centimeters (top) measured from the base. Some of these properties were also determined in the intact leaves of 4-day-old maize plants.     

Proteinaceous factor reactivating an inactive form of pyruvate Pi dikinase isolated from dark-treated maize leaves

A proteinaceous factor required for the Pi-dependent reactivationof the inactive form of pyruvate Pi dikinase from dark-treatedmaize leaves, was isolated and partially purified by SephadexG-200 gel filtration. The factor, which is sensitive to trypticdigestion and heat-treatment, showed enzyme-like kinetic behaviourin the activation reaction. ¹Present address: Laboratory of Biochemistry, Department ofAgricultural Chemistry, School of Agriculture, Shizuoka University,836 Ohya, Shizuoka 420, Japan. (Received February 25, 1974; )     

Comparison of the metabolic properties of plastids isolated from developing leaves or embryos of Brassica napus L

Plastids isolated from developing leaves and embryos of oilseed rape (Brassica napus L.) were incubated with substrates in the light or the dark, with or without exogenous ATP. Incorporation of HCO^-3, and carbon from a range of substrates into fatty acids and/or starch by leaf chloroplasts was absolutely light-dependent and was unaffected by provision of ATP. Incorporation of HCO^-3 into fatty acids and/or starch by embryo plastids was also light-dependent. However, the light-dependent rates attained, when expressed on a comparable basis, were less than 32% of those from Glc6P (plus ATP), which was the most effective substrate for starch and fatty acid synthesis. In the light alone the rates of carbon incorporation from Glc6P, pyruvate and acetate into fatty acids, and from Glc6P into starch by embryo plastids were less than 27% of the respective ATP-dependent (dark) rates. Light had no effect on these ATP-dependent rates of synthesis by embryo plastids. While transporter activities for both glucose and Glc6P were present in embryo plastids, leaf chloroplasts did not have the latter activity. It is concluded that light at in vivo levels can contribute energy to carbon metabolism in embryo plastids. However, this contribution is likely to be small and these plastids are therefore largely dependent upon interaction with the cytosol for the ATP, reducing power and carbon precursors that are required for maximal rates of starch and fatty acid synthesis.     

Light-induced structural changes during incubation of isolated maize etioplasts

Summary Etioplasts were isolated from leaves of 9-day-old etiolated maize (Zea mays L.) seedlings and incubated in a relatively simple medium in light and in the dark. During the first 5 h no changes occurred in the fine structure of the isolated etioplasts in the dark. In light the size of the prolamellar bodies decreased and significantly more plastid sections without prolamellar bodies were counted. The total length of the thylakoids per plastid section increased, but there was no evidence for bi- and polythylakoid formation. It is concluded that light induces the structural transformation of the prolamellar body membranes into primary thylakoids also in isolated etioplasts.